Who's that a threat to? I know that if any of these witnesses told me the above as a friend and I learned how they and others died, I'd be terrified. That was the most likely point of the murders, but that still leaves out "why?" 

After a couple minutes of pondering, there is only one answer that holds water. What happens if these people come back with another USSR-linked Lee Harvey Oswald trying to kick off WWIII? Sure enough they did, in 1968, they had the Palestinian-born Sirhan Sirhan assassinate RFK just before the elections. (That same day they just missed taking out Andy Warhol for effect.)

For all we know, the movie JFK was released during the Bill Clinton election campaign because they knew his probable fate. Having made that more difficult, they still got at Clinton, a sex addict, with the pornography made flesh by the name of Monica Lewinsky.